The field to which this invention pertains is generation and regeneration of a rhodium containing solution. This solution can be used as a catalyst precursor for the industrial manufacture of acetic acid, acetic anhydride, ethylidene diacetate and related processes utilising compounds of rhodium.
For the industrial production of acetic acid, carbonylation of methanol has long been the preferred method. Dimethyl ether and methyl acetate and mixtures of these two components may also be used as feed, optionally admixed with methanol. These reactions are catalysed by certain transition metals together with iodide in the form of methyl iodide and/or hydriodic acid. Of the transition metals, rhodium and iridium are preferred due to their high activity and selectivity. In industrial plants, the metal catalyst exists in a dissolved form during operation of the plant. In those cases, where the catalyst is rhodium, it is generally recognised that the predominant rhodium containing specimen present under the conditions of operation is the anionic complex [Rh(CO)2I2]xe2x88x92. This complex is formedxe2x80x94in the form of the acid or as a saltxe2x80x94from virtually any rhodium source under the conditions of operation (at a temperature above 150xc2x0 C. and at least 5 bar (5xc2x7105 Pa) CO-pressure).
The currently preferred rhodium source for start-up of acetic acid and acetic anhydride plants is rhodium iodide, RhI3, which in pure form appears as a black solid. It is not desirable to transfer it to the reactor zone in an undissolved form, since the dissolution of this compound under the conditions of operation takes some time, and several complications may occur before the dissolution is complete. Rhodium iodide, however, is well known to be practically insoluble at ambient conditions. Thus water, acetic acid, methanol, methylacetate and other common solvents have no effect on RhI3. Although it has some solubility in hydriodic acid, the solubility is low, and very high concentrations of hydriodic acid are necessary. Other rhodium compounds are more soluble, e.g. RhCl3, but they precipitate upon admixture with solutions of hydriodic acid due to reaction with the iodide ions present in the solution. Thus, since iodide is needed as a co-catalyst, RhI3 precipitates in the reactor upon admixture of a RhCl3 solution with the iodine containing co-catalyst solution. Certain complexes of rhodium are known to be more stable in the presence of iodide, but they are not preferred since they are generally costly and difficult to handle. Some of these compounds furthermore contain elements or molecular entities, the presence of which may be undesirable. Examples are sulphur, chlorine, bromine and arsenic containing complexes. Even though rhodium iodide may react with reducing agents and thus be brought into solution, such solutions are often sensitive towards air, which may re-oxidise rhodium and thus reprecipitate RhI3. Another problem pertains to rhodium being fairly noble, thus the admixture of rhodium iodide with strong reducing agents may potentially cause over-reduction to elemental rhodium.
It is well known that dissolution of RhI3 is accomplished under certain circumstances by reduction in the presence of a solvent. Such reduction may be carried out with hydrogen or carbon monoxide or synthesis gas at elevated temperatures and pressure, in which case the active form of the catalyst [Rh(CO)2I2]xe2x88x92 is formed directly according to the equation
RhI3+3 CO+H2O=H[Rh(CO)2I2]+CO2+HI 
The solution thus obtained is stable towards high concentrations of iodide, but only as long as a minimum pressure of carbon monoxide is maintained and only as long as air and other oxidising agents are avoided. This is the method of current practice in the art of catalyst generation for rhodium catalysed acetic acid production.
Obviously, this method is tedious and costly since the catalyst solution must be prepared under pressure and with applied heat and also must be transferred to the methanol carbonylation reactor under pressure. Furthermore, it is well known that rhodium iodide (probably in an impure form) precipitates in certain parts of the internals of acetic acid plants during operation. Due to the very high price of rhodium, it is feasible to regenerate the catalyst from these solid precipitates. Regeneration of the catalyst solution from the solid may be carried out in the same way as described above for start-up.
A more preferable catalyst formulation would be a solution containing rhodium and iodide in high concentrations. This solution should be stable towards precipitation of RhI3, stable on exposure to air and water, and should not contain undesired elements such as sulphur, arsenic and the like. Such a catalyst formulation would allow for easy and less expensive transportation and simple transfer to the reactor.
As explained above, however, such a catalyst formulation is very hard to achieve. It was therefore highly surprising to discover that solid RhI3 dissolves with hydrazine hydrate and particularly that solutions with a very high concentration of rhodium can be formed and are stable for at least several months.
The present invention thus provides a method for generating a concentrated solution of rhodium starting from solid RhI3 and other solid rhodium sources by combination with a reducing agent, preferably hydrazine and hydrazine derivatives. Said solution does not form any precipitate of rhodium iodide or other compounds even upon admixture with large quantities of hydriodic acid. Said solution is stable at ambient conditions, very stable towards air and is easily prepared without application of external pressure and heating sources.
The use of hydrazine and other reducing agents for the activation of rhodium containing catalysts has been claimed in a number of patents: JP 63/227531 A, JP 87/60062 A, JP 62/148437 A, JP 85/289269 A, JP 88/041892 B, JP 56/144747 A, DE 3,115,032 A, U.S. Pat. No. 4,376,724 A, JP 83/018147 B, U.S. Pat. No. 4,420,420 A, DE 3,115,032 C. The known technique, however, claims use of a reducing agent for activating a solid (heterogeneous) rhodium containing catalyst without the catalyst being dissolved. Obviously, it is not the intention of the processes described in the above patents to prepare rhodium containing solutions.
Some other patents claim the use of rhodium and hydrazine in combination to form homogeneous catalysts. Thus, NO 169342 B, DK 164815 B and U.S. Pat. No. 4,550,096 teach the preparation of homogeneous hydrogenation catalysts and U.S. Pat. No. 5,051,522 teaches the use of hydrazine to prepare a hydroformylation catalyst. None of these patents pertain to the field of methanol carbonylation, however, and the stabilities of the rhodium catalysts toward iodide are not discussed.
A process for recovering Group VIII noble metals by extraction with amines, including the use of hydrazine, is claimed in U.S. Pat. No. 4,341,741. According to this patent, noble metals such as rhodium and iridium used in the carbonylation of methyl acetate and dimethyl ether, accumulate in a residue formed during the carbonylation reaction, containing typically 0-4% wt/wt rhodium. A sample containing 1% Rh was at first partly extracted with dilute hydrochloric acid and methylene chloride, leading to a 0.5% Rh content in the residue. Ten (10) miligram of the acid extracted residue was then dissolved in 5 ml methyl acetate and treated with 0.1 ml of hydrazine hydrate. It was demonstrated that more than 85% of the residue-bound Rh was extracted this way. However, the concentration of rhodium in the extract was no more than 0.01 g/l or less than 0.1xc2x710xe2x88x923 M. Typical rhodium concentrations in an acetic acid plant reactor are 1.0xc2x710xe2x88x923 M, and a catalyst solution should preferably be even more concentrated to facilitate transport and start-up.
The rhodium containing solutions of the present invention carry up to as much as 0.34 M rhodium, and have furthermore been demonstrated to be stable for long periods of time. Even by addition of an iodide source, a high concentration of rhodium (at least 0.03 M) can be maintained.
The invention concerns a method for generating a solution containing rhodium at a concentration of at least 0.01 M, obtained by treating solid RhI3 or other solid source of rhodium with a reducing agent selected from a source of hydrazine, hydrazine hydrate, hydrazinium salts, organic derivatives of hydrazine, hypophosphorous acid and salts of hypophosphorous acid. Optionally, a co-solvent can be added, at normal pressure and without the use of carbon monoxide and synthesis gas.
The invention also concerns the use of the method stated above, for the regeneration of rhodium catalyst from industrial plants producing acetic acid and derivatives thereof.
The invention concerns moreover a solution containing rhodium at a concentration of at least 0.01 M, which is preparable by the method mentioned above.
Another aspect of the invention concerns the use of the solution mentioned above as a catalyst source.
Finally, it is also the intention to provide a product which is made available by isolation of a rhodium containing solid from a solution obtained by the method according to the invention.
It is the intention of the present invention to provide a method for the generation and regeneration of a solution containing rhodium at a concentration of at least 0.01 M. The method of the present invention comprises steps of treating solid rhodium (III) iodide or another rhodium containing solid with a reducing agent under ambient conditions, thereby causing complete dissolution of the rhodium source. This operation may beneficially be carried out with the rhodium containing solid suspended in water, acetic acid or another solvent before addition of the reducing agent and optionally the complexing agent.
As reducing agents, hydrazine and hypophosphorous acid have the desired effect of dissolving rhodium iodide. Hydrazine is presently particularly preferred since complete and instant dissolution of RhI3 is accomplished without the need for an external source of heat (the reaction between rhodium iodide and hydrazine is exothermic). Hydrazine in the reactor will not alter activity or selectivity. During the conditions of operation it will most likely react to ammonium iodide and primary, secondary, tertiary and quaternary methyl ammonium iodide salts. These salts have a high solubility in the reaction medium and will not alter the process in the low amounts needed to generate and regenerate rhodium solutions. In the following examples, hydrazine hydrate has been used as the source of hydrazine.
Thus, the preferred method of this invention for generating a rhodium containing solution consists of treating the solid rhodium source (e. g., RhI3) with hydrazine hydrate; optionally admixed with a co-solvent. Other solid sources of rhodium include rhodium chloride, rhodium bromide, rhodium nitrate, rhodium acetate, rhodium oxide, and rhodium sulphate.
This method is inexpensive and easy to carry out. The method provides a rhodium containing solution, which is infinitely stable towards air under ambient conditions, which does not precipitate rhodium iodide upon addition of hydriodic acid or other iodide sources and which may contain rhodium in concentrations of at least 0.01 M. Furthermore, the solution does not contain elements, which are not usually present in acetic acid plants apart from small amounts of nitrogen, which does not cause any damage or problems during operation of such plants.
Thus, one part of RhI3 (by weight) is instantly dissolved upon admixture with 2 parts (by weight) of hydrazine hydrate to give an orange solution. The dissolution happens without application of heat apart from that evolved in the course of the reaction. Provided a small excess of hydrazine is present, this solution does not form a precipitate even upon admixture with a large amount of hydriodic acid.
A practical application of the invention would be to use a solution containing a reducing agent selected from hydrazine, hydrazine derivatives, hydrazinium salts, hypophosphorous acid and hypophosphite salts, to dissolve rhodium containing precipitates in the internals of an acetic acid plant, without removing the precipitate from the location inside the plant.
The rhodium solution obtained by the method according to the invention can also be used in a process for the production of both carboxylic acids such as, for example, acetic acid, and for their derivatives, whereby the corresponding alcohol is carbonylated. This would involve a step, whereby the carboxylic acid is contacted with the corresponding alcohol in the presence of the rhodium solution, which thereby acts as a catalyst source for the reaction.